Synergetic Kinetics of High-Temperature Surface Degradation Processes in Austenitic Heat-Resistant Cast Steels

When the severity of high-temperature environments increases, the oxidation mode changes from a normal oxidation mode, when a formed oxide layer protects the surface of austenitic heat-resistant cast steels, to an extreme mode, when scale spallation and partial vaporization intensify surface degradation. These processes can also influence each other. On the example of two oxidized austenitic heat-resistant steels with different alloying levels, we show how the described simulation/experimental methodology may be applied to analyze and quantify the synergetic kinetics of the extreme surface degradation processes. The surface degradation processes were formulated using diffusion-controlled parabolic oxidation, time-dependent vaporization, and spallation with an intensity cross-linked to the instantaneous thickness of the oxide layer. The 400 h tests were performed in air and water vapor-containing combustion gases at upper working temperatures. Three experimentally obtained surface degradation quantities included changing the weight of the specimen with adherent oxide, the weight of the spalled scale, and the thickness of the formed adherent oxide. These experimental quantities were used to compute the values of oxidation, vaporization, and spallation kinetic constants for each uninterrupted oxidation test. An additional interrupt every 100-h test was also performed to verify the model-predicted trends. It was shown that the alloying level in austenitic heat-resistant cast steel determines its ability to withstand surface degradation at extreme oxidation mode by the synergetic effects of oxidation, vaporization, and spallation. The described approach can be used for the metallic component lifetime prediction in severe gaseous environments.